I have a server V880 on which the clock is jumping suddenly
for an example
when I set the date to
Sun 22 Jul 16:05:56 EEST 2012
it just suddenly after a while change to 19:45:46
Suddenly
so
Can any one tell me what are the possible causes for that issue ???

You need to determine whether you have a software issue or a hardware issue.

Hardware:
If the time setting always reverts to the same moment in eternity such as something like January 1 of 1970 (Unix epoch time) or something like November 9 of 2001 (first ship date of the V880), then the 3v battery on the I/O board or else the 3v battery on the RSC board may be failing. A weak battery couild also get you a wandering clock if the I/O board and the RSC try to sync to each other.

Software:
If the jump in time is always the same interval such as a consistent three hours from what you manually set it to then you have a misconfigured timezone/locale issue or you could have a broken or misdirected setting to the NTP server.

My suspicion? Softtware.
NTP settings have changed and that other system is returning an incorrect locale to youe V880